My guess would be a seat from a Plymouth Belvedere/Satellite of the same age might be a similar enough size to fit yours, if you cant get one from a Coronet. Whilst they may both be 66 B-bodies, it doesnt guarantee they'll both fit

I've sent an email out to a bunch of 62-65 owners via the http://www.1962to1965mopar.ornocar.com/welcome.html website asking if anyone knows what other seat(s) might fit - I'll let you know if anyone's got any bright ideas.